Known fixing methods of an electrophotographically formed toner image include heat fixing and pressure fixing. Heat fixing requires a rise time for a fixing means to be heated up to a prescribed temperature after turning on the power and afterward energy for maintaining the fixing means at that temperature. Pressure fixing, on the other hand, requires neither a rise time nor heating energy. However, fixing strength obtained by pressure fixing is not sufficient so that high pressure should be applied for providing a toner image resistant to rubbing.
Capsule toners comprising a core material containing a fixing component, and a hard outer shell, which completely covers the core material, have been developed for solving the above-mentioned fixing problems. The outer shell of a capsule toner is destroyed on pressure application, and the fixing component runs off and is fixed on an image-receiving medium, such as paper and an OHP sheet.
Capsule toners may contain magnetic particles for imparting magnetic characteristics or colorant particles for coloring. However, if a large amount of magnetic particles or colorant particles is incorporated into the capsule core material, the core material will have a so increased viscosity, and the fixing component fails to run off on pressure application, only resulting in poor fixing.
In order to improve fixing properties of capsule toners, it is necessary that a major proportion of the colorant particles or magnetic particles be present in the shell. However, if these particles are exposed on the surface of the shell, the resistivity of that part decreases to reduce insulating properties of the capsule surface, which may cause leakage of electric charge, resulting in a reduction in charge quantity. Where the colorant particles or magnetic particles are hydrophilic, the charging properties become largely dependent on environmental conditions. Therefore, it is important that the colorant particles or magnetic particles should not be exposed on the shell surface.
If the fixing component oozes out of capsules during preparation or storage, the toner undergoes agglomeration or considerable reduction in fixing properties. Therefore, the outer shell of capsule toners must block penetration of the fixing component. From this viewpoint, however, the conventional capsule toners are not always satisfactory.
For example, JP-A-57-179860 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a capsule toner whose outer shell comprises polyurethane or polyurea prepared by the reaction of an isocyanate, water, and an amine or glycol component, and JP-A-60-222866 discloses a capsule toner whose outer shell comprises polyurethane or polyurea prepared by using one bifunctional isocyanate and at least one tri- or polyfunctional isocyanate. In cases where the amount of magnetic particles to be incorporated is increased or where the shell thickness is decreased for capsule size reduction, the colorant particles or magnetic particles are ought to have an increased concentration in the shell and be exposed on the shell surface. As a result, the quantity of charge is reduced, environment dependence of charging properties is increased, or the colorant particles or magnetic particles pierce the shell, through which the fixing component, particularly a low-molecular-weight component would run out.